Field of the Invention
The present invention relates to a method for controlling the electron current flowing in an x-ray tube in the form of an electron beam propagating between an electron emitter and an anode, the electron emitter having a focussing electrode and being continuously heated during the operation of the x-ray tube the electron beam striking the anode in a focal spot, with the tube voltage being across the electron emitter and the anode, and the focal electrode being at a focussing electrode potential. The invention also relates to an x-ray system operating according to this method.
In contemporary x-ray tubes, continuously heated tungsten helical coil is employed as the electron emitting component almost exclusively. The tube current--i.e. the electron current emanating from the electron emitter given a defined tube voltage--is therein determined by the temperature of the helix, which is adjusted by the heating current through the tungsten helix. Because of the low heating capacity of the tungsten helix it is possible to rapidly alter the tube current while maintaining the respective size of the focal spot by altering the heating current, which is necessary for many medical recording techniques. In continuously heated low-temperature emitters which are fashioned from materials--e.g. LaB.sub.6 --with a lower specific electron work function than tungsten and which as a rule have a significantly higher heating capacity than tungsten, alterations of the tube current at the filament are not possible with the same speed as with a tungsten helix, which is why low-temperature emitters cannot be employed everywhere. In many modern x-ray tubes--e.g. rotating bulb tubes with central emitters or x-ray tubes with oblique bombardment--round emitters with a small emission surface and a high emission current are needed to generate an electron beam with an approximately circular cross-section. The known tungsten helices are unsuitable for these tube geometries. The low-temperature emitters that are otherwise suitable, however, cannot bear rapid temperature changes such as are necessary for medical recording techniques with rapidly varying tube current. If a low-temperature emitter should be employed despite this, then the controlling of the tube current--i.e. the adjustment of the electron current--must occur in a different way than by alteration of the heating current. This can be effected by an additional electrode, for example a grid connected upstream, a Wehnelt cylinder or a focussing electrode at a different potential than that of the electron emitter. A disadvantage of such approaches, however, is that the potential distortion brought about by the additional electrode simultaneously influences the spread of the electron beam such that the abovementioned arrangement is only suitable for turning the electron current, and thus the tube current, on and off in alternation, but is not suitable for variable control without simultaneously adversely influencing other focussing, and thus the size of the focal spot dependent on the potential at the additional electrode, and thus on the tube current.
An x-ray tube having the capability of adjusting of the tube current but without any consideration of the tube voltage and/or the size of the focal spot, known from U.S. Pat. No. 5,617,464.